Scientia Agricultura Sinica ›› 2022, Vol. 55 ›› Issue (6): 1110-1126.doi: 10.3864/j.issn.0578-1752.2022.06.005

• TILLAGE & CULTIVATION·PHYSIOLOGY & BIOCHEMISTRY·AGRICULTURE INFORMATION TECHNOLOGY • Previous Articles     Next Articles

Early Detection on Wheat Canopy Powdery Mildew with Hyperspectral Imaging

CAI WeiDi(),ZHANG Yu,LIU HaiYan,ZHENG HengBiao,CHENG Tao,TIAN YongChao,ZHU Yan,CAO WeiXing,YAO Xia()   

  1. College of Agriculture, Nanjing Agricultural University/National Engineering and Technology Center for Information Agriculture/ Engineering Research Center of Smart Agriculture, Ministry of Education/Key Laboratory for Crop System Analysis and Decision Making, Ministry of Agriculture and Rural Affairs/Jiangsu Key Laboratory for Information Agriculture/Collaborative Innovation Center for Modern Crop Production Co-sponsored by Province and Ministry, Nanjing 210095
  • Received:2021-05-25 Accepted:2021-09-06 Online:2022-03-16 Published:2022-03-25
  • Contact: Xia YAO E-mail:2019101180@njau.edu.cn;yaoxia@njau.edu.cn

Abstract:

【Objective】In this study, the near-ground imaging spectrometer was used to obtain time-series images of wheat canopy after inoculation with powdery mildew, which aimed to explore the ability and performance of the combination of spectral feature and texture feature in the early detection of wheat powdery mildew at canopy scale. 【Method】 Based on the field trials of wheat varieties with different disease resistance in different years, the wavelet features sensitive to wheat powdery mildew were extracted by continuous wavelet transform (CWT) method, and the corresponding texture features were extracted based on spectral features to construct normalized difference texture index (NDTI). Meanwhile, the representative traditional vegetation indices (Vis) were selected. Then, based on these features and combinations, the partial least squares discriminant analysis (PLS-LDA) model was used to establish wheat canopy healthy and disease recognition model. The partial least squares regression (PLSR) was used to estimate the severity of wheat canopy disease. The technique was used to distinguish the healthy and disease wheat at different days after inoculation based on the optimal features and combinations. 【Result】 Based on CWT, the selected four wavelet features were 595 nm (yellow region) at 6 scales, 614 nm (red region) at 5 scales, 708 nm (near infrared region) at 3 scales, and 754 nm (near infrared region) at 4 scales respectively. The following texture features were selected for the best texture index combination: ENT754, MEA754, ENT708, ENT595, ENT614, HOM708, HOM595, HOM614, DIS595, HOM754 and DIS614. Besides, it was found that the texture feature MEA754 had the superior performance among all the texture, with the highest correlation between the severity of disease and texture (R2=0.67). The PLS-LDA model based on the combination of wavelet feature and texture feature had the highest accuracy, with the overall classification accuracy of 81.17% and the Kappa coefficient of 0.63. In addition, the PLSR model based on spectral index and texture index was the best, and the R 2 of modeling and testing was 0.76 and 0.71, respectively. The severity of wheat canopy powdery mildew was about 26% (about 24 days after inoculation), which was identified in this study at the earliest time. 【Conclusion】 The wheat healthy and disease recognition model based on the combination of wavelet feature and texture feature could significantly improve the accuracy of disease classification, and the combination of spectral index and texture index could significantly improve the accuracy and stability of disease severity estimation. The method and results of this study could provide the reference for disease monitoring of other crops and technical support for accurate application of modern intelligent agriculture.

Key words: wheat powdery mildew, canopy, hyperspectral imaging, continuous wavelet transform, texture feature

Fig. 1

Canopy test site"

Fig. 2

Canopy test"

Table 1

Time of obtaining canopy data"

2018 测试日期 Test date
04-09 04-17 04-19 04-25 04-28 05-03 05-09
接种后天数DAI 8 16 18 24 27 32 38
样本数量Sample number 14 13 14 14 14 14 13
2017 测试日期 Test date
03-27 04-01 04-13 04-22 04-28 05-10
接种后天数DAI 6 11 23 32 38 50
样本数量Sample number 6 6 10 12 12 12

Table 2

Texture features based on the second-order statistics filter"

名称Name 函数表达式Function expression 描述Description
均值 Mean, MEA $\sum_{i, j=0}^{N=j} i \times P_{i, j}$ 反映了灰度平均值情况 Reflect the gray mean value
方差 Variance, VAR $\sum\limits_{i,j=0}^{N-1}{i\times {{P}_{i,j}}{{(i-MEA)}^{2}}}$ 反映了灰度变化的大小Reflect the change of grayscale
均一性 Homogeneity, HOM $\sum_{i=0}^{N-1} \sum_{j=0}^{N-1} P_{i, j} P_{i, j} /\left[1+(i+j)^{2}\right]$ 反映了纹理局部同质性Reflect local homogeneity of texture
对比度 Contrast, CON $\sum_{n=0}^{N-1} n^{2}\left\{\sum_{i=0}^{N-1} \sum_{\substack{j=0 \\|i-j|=n}}^{N-1} P_{i, j}\right\}$ 反映了纹理的清晰度Reflect texture sharpness
异质性 Dissimilarity, DIS $\sum_{i, j=0}^{N-1} i \times P_{i, j}|i-j|$ 反映纹理的相似性Reflect texture similarity
熵 Entropy, ENT $\sum_{i=0}^{N-1} \sum_{j=0}^{N-1} P_{i, j} \log P_{i, j}$ 反映图像具有的信息量Reflect the amount of image information
角二阶矩 Second Moment, SEM $\sum_{i, j=0}^{N=j} i \times P_{i, j}^{2}$ 反映了图像灰度分布的均匀性
Reflect the uniformity of image gray distribution
相关性 Correlation, COR $\left[\sum_{i=0}^{N-1} \sum_{j=0}^{N-1} i j P_{i, j}-u_{1} u_{2}\right] / \sigma_{1} \sigma_{2}$ 反映某种灰度值沿某个方向的延伸长度
Reflect the extension length of some gray value along a certain direction

Table 3

Vegetation indices used in this study"

定义
Definition
方程
Equation
相关生理参数
Relative physical parameter
白粉病指数 Powdery mildew index, PMI (R515-R698)/(R515+R698)-0.5×R738 小麦病害Wheat disease[49]
简单修改比 Modified simple ratio, MSR (R800/R670 -1)/ (R800/R670 + 1)1/2 叶面积Leaf area[50]
光化学反射指数 Photochemical reflectance index, PRI (R570- R531)/ (R570 + R531) 光合辐射Photosynthetic radiation[51]
光合辐射 Photosynthetic radiation, PhRI (R550 - R531)/ (R550 + R531) 光合利用效率Light use efficiency[51]
改进的叶绿素吸收比指数
Modified chlorophyll absorption ratio index, MCARI
[(R701-R671)-0.2(R701-R549)]/(R701/R671) 叶绿素吸收Chlorophyll absorption[52]
花青素反射指数 Anthocyanin reflectance index, ARI R550-1-R700-1 花青素含量Anthocyanin content[53]
与结构无关的色素指数
Structure independent pigment index, SIPI
(R800 - R445)/(R800 - R680) 色素含量Pigment content[54]
归一化色素叶绿素比值指数
Normalized pigment chlorophyll ration index, NPCI
(R680 - R430)/(R680 + R430) 叶绿素比值Chlorophyll ratio[54]
红边植被胁迫指数 Red-edge vegetation stress index, RVSI [(R712 + R752)/2] - R732 生物量Biomass[55]

Fig. 3

Changes and comparison of canopy chlorophyll content (CCC) and canopy water content (CWC) between healthy and infected canopy tested at 2018 and 2017 a represents no significant difference; b represents significant difference (P<0.05). A, B represent 2018; C, D represent 2017"

Fig. 4

Spectral reflectance time-series changes of healthy and infected wheat canopy 8 d (4%) represents the data acquired at 8th day after innoculation and the average disease severity of infected canopy is 4% and the rest are similar"

Fig. 5

Correlation scalogram of continuous wavelet coefficients with DI at different scales The scalogram shows the determinant coefficient (R2) of the wavelet coefficients and DI. The red part represents the top 5% R2 region"

Table 4

Wavelet features based on selected disease-sensitive wavelet region selection"

光谱区域
Spectral region
入选波段
Selected wavelength
尺度
Scale
决定系数
R2
黄光区域Yellow region 595 6 0.82
红光区域Red region 614 5 0.83
近红外区域Near infrared region 708 3 0.85
近红外区域Near infrared region 754 4 0.84

Fig. 6

The texture features contrast images of healthy and infected canopy at 8th day and 38th day after inoculation a: Healthy, b: Infected, c: Healthy, d: Infected. MEA: Mean; VAR: Variance; HOM: Homogeneity; CON: Contrast; DIS: Dissimilarity; ENT: Entropy; SEM: Second moment; COR: Correlation"

Table 5

The top ten best-performing normalized difference texture indices correlated with DI"

模型
Model
纹理指数
Texture index
入选纹理 Selected texture 决定系数
Determinant (R2)
T1 T2
线性模型
Linear model
NDTI
(T1, T2)
ENT754 MEA754 0.51
MEA754 ENT708 0.50
MEA754 ENT595 0.50
MEA754 ENT614 0.50
MEA754 HOM708 0.47
MEA754 HOM595 0.47
MEA754 HOM614 0.46
HOM754 MEA754 0.46
MEA754 DIS595 0.46
MEA754 DIS614 0.45

Table 6

Linear relationships between texture features and DI"

纹理特征
Texture feature
小波特征 Wavelet feature
595 nm 614 nm 708 nm 754 nm
均值 MEA 0.00ns 0.00ns 0.01ns 0.67***
方差 VAR 0.29*** 0.31*** 0.29*** 0.01ns
均一性 HOM 0.25** 0.23** 0.27** 0.27**
对比度 CON 0.27*** 0.29*** 0.27*** 0.01ns
异质性 DIS 0.28** 0.30*** 0.28*** 0.08**
熵 ENT 0.30ns 0.26ns 0.35ns 0.38ns
角二阶矩 SEM 0.28*** 0.27*** 0.28*** 0.28***
相关性 COR 0.01ns 0.02ns 0.03ns 0.01ns

Table 7

Results of wheat healthy and infected canopy identification model with different features"

输入特征
Input feature
特征数量
Number of features
分类精度 Classification accuracy (%) 总体分类精度
OAA (%)
卡帕系数
Kappa
健康 Healthy 感病 Infected
小波特征与纹理特征结合 WFs & TFs 36 92.54 72.41 81.17 0.63
光谱指数 VIs 9 89.55 70.11 78.57 0.58
光谱指数与纹理指数结合 VIs & NDTIs 19 86.57 71.26 77.92 0.56
纹理特征 TFs 32 85.07 71.26 77.27 0.55
纹理指数 NDTIs 10 77.61 70.11 73.38 0.47
小波特征 WFs 4 82.09 64.37 72.08 0.45

Table 8

Performance of PLSR model with different features"

输入特征
Input feature
特征数量
Number of features
建模 Calibration 检验 Validation
R2 RMSE R2 RMSE RRMSE
小波特征与纹理特征结合WFs & TFs 36 0.8 8.13 0.68 11.84 0.39
光谱指数VIs 9 0.76 8.70 0.69 11.54 0.38
光谱指数与纹理指数结合VIs & NDTIs 19 0.76 8.89 0.71 11.30 0.38
纹理特征TFs 32 0.72 9.42 0.42 15.94 0.53
小波特征WFs 4 0.64 9.72 0.61 12.93 0.43
纹理指数NDTIs 10 0.59 9.85 0.47 15.31 0.51

Fig. 7

Cross-validation 1:1 scatter plots for measured DI versus estimated DI derived from selected models a : WFs, b : TFs, c : WFs & TFs, d : VIs, e : NDTIs, f : VIs & NDTIs"

Table 9

Discriminant results based on PLS-LDA and combination of wavelet features and texture features"

年份
Year
分类精度 Classification accuracy (%)
8d(7.4%) 16d(15.9%) 18d(26.2%) 24d(27.7%) 27d(32.3%) 32d(41.7%) 38d(63.2%)
2018 健康Healthy 83.33 100 83.33 100 100 100 100
感病Infected 75.00 100 75.00 100 100 100 100
Kappa 0.57 0.56 0.57 0.84 1 1 1
6d(1.5%) 11d(8.9%) 23d(25.2%) 32d(26.2%) 38d(33%) 50d(33%)
2017 健康Healthy 66.67 61.67 71.67 100 100 100
感病Infected 66.67 75.00 77.50 100 100 100
Kappa 0.33 0.5 0.55 0.85 1 1
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